With recent requirement for ever decreasing sizes of semiconductor devices, it has become necessary to more finely form a circuit pattern on a surface of a semiconductor wafer. In order to form such a fine circuit pattern, in the manufacturing process of semiconductor devices, it is required to accurately examine a surface structure of a wafer in which a plurality of films is stacked, e.g., a wafer of each film after an etching process is performed.
Conventionally, after the wafer is etched, a surface structure of the wafer is examined by observing and photographing the cross section of the cleaved wafer with a scanning electron microscope (SEM). To employ such a method, it is inevitable to cut the wafer to observe the cross section of the wafer.
Accordingly, to examine the surface structure of the wafer without destructing the wafer after the etching process is performed, there has recently been developed a method of applying scatterometry such as elipsometry or reflectometry, which has been used for evaluation of a resist pattern or the like, to examination of the surface structure of a wafer (see, e.g., Japanese Patent Laid-open Application No. 2002-260994).
Especially, by employing the reflectometry as the scatterometry, it is possible to examine the surface structure of the wafer without destructing the wafer by using optical constants (a refractive index n and an attenuation constant k) of the surface structure of the wafer. In detail, the optical constants n and k of each film stacked on the surface of a wafer, e.g., a gate oxide film, a polysilicon film, an oxide film, a bottom anti-reflective coating (BARC) film, and a photoresist film, are estimated. Next, optical models representing surface structures, e.g., groove patterns of the wafer are developed and stored for various groove patterns by using the estimated optical constants of the respective films. Then, the surface structure (e.g., a groove pattern) of the wafer is examined by measuring a surface reflectance of the wafer and selecting a model of the groove pattern corresponding to the reflectance (see, e.g., Japanese Patent Laid-open Application No. 2005-033187).
Accordingly, in the scatterometry, when the optical constants of the respective films are not accurately obtained, it may be difficult to accurately examine the surface structure of the wafer. As a result, it becomes very important to accurately obtain the optical constants of the respective films.
A so-called fitting method is conventionally employed to obtain the optical constants. In the fitting, a white beam is emitted to an object film, and a reflected beam of the white beam is measured to obtain an actually measured spectrum of a spectral reflectance. Then, in the fitting, the optical constants are obtained by employing a theoretical model to fit the actually measured spectrum, wherein a set of fitting parameters of the model which is used to produce the optical constants is determined such that the set gives the theoretical model a best fit to the measured actually measured spectrum. At the time when the  fitting is performed, it is possible to reduce the number of parameters by fixing optical constants of a below-located film that is located below a target film with values obtained previously, thereby decreasing the obtaining time.
However, if previously obtained optical constants of a below-located film that is located below a target film are used in the obtaining process of the optical constants of the respective films, even though the optical constants of the below-located film have been obtained through the satisfactory fitting, optical constants of the target film located above the below-located film may be obtained through an unsatisfactory fitting. This may deteriorate the accuracy in the optical constants of the film.
In the fitting, the optical constants are fundamentally obtained by acquiring a combination of parameters that minimizes the difference between the actually measured spectrum and the theoretical spectrum. Accordingly, since the optical constants are obtained when the theoretical spectrum fits best to the actually measured spectrum, it is considered that the optical constants are adequate.
However, since a plurality of unknown parameters is changed in the fitting, the more the number of the unknown parameters, the more their combinations there are. Accordingly, there may be a plurality of local minimum solutions (i.e., combinations of the parameters) to minimize the difference between the actually measured spectrum and the theoretical spectrum. In this case, even though certain local minimum solutions, i.e., certain combinations of the parameters are mathematically correct, all the certain combinations may not be physically correct. If the optical constants of the below-located film are not physically correct, when the optical constants of the target film located above the below-located film are obtained by using the physically incorrect optical constants, it is highly likely that the fitting is unsatisfactorily performed.